AN224| Application Note
Maxim > App Notes > BATTERY MANAGEMENT Keywords: battery monitor, current measurement, offset, DS2761, offset correction, calibration, DS2760, DS2751
Oct 25, 2002
APPLICATION NOTE 224
Calibrating the Offset Register of the DS2761
Abstract: The DS2761 contains a current offset register that can be utilized by the designer to eliminate inherent offset introduced by the current A/D within the IC. Improper calibration of this register, however, can negate any positive effect it has on increasing the accuracy of the current measurements, especially at low currents. This application note describes to the reader a step-by-step approach on how to properly calibrate the device within the fully assembled battery pack in order to determine the correct value that should be placed into the current offset register of the DS2761. Although the application note is specifically written for the DS2761, it also is applicable to the DS2760 and DS2751.
Introduction
The advantages of lithium-ion (Li+) protection and high accuracy fuel gauging can be completely negated by an assembly error or improper calibration of the device. This application note walks through an example of how to properly calibrate the offset of the DS2761's high accuracy current A/D.
Calibrating the Current Offset
The Current A/D of the DS2761 is extremely sensitive. It is capable of measuring a voltage drop of only 15.625uV across the sense resistor. This kind of accuracy can only be achieved by calibrating the current measurement after the cell pack is assembled. The Current Offset Register (Address 0x33h) allows the current measurements of the DS2761 to be adjusted by 127 LSbs ( 79.375mA or 1.984mV) for accurate measurement of very small currents. This offset is subtracted internally from each current measurement and is reflected in the Current Register and the Accumulated Current Register. There can be slight variations in the current offset of the device across the temperature and voltage range of the application. Therefore, it is recommended to calibrate the offset at the average temperature and voltage of the application. For example, a cell phone or PDA would spend the majority of its time at approximately 25 C and 3.8V, which would be room temperature and the mid-range of the cell voltage. The following exa
mple lists the steps to calibrate the DS2761 in-circuit:
1. Initialize the Offset Register
It is recommended to start by writing the Offset Register to 0x00h. The user may select a different offset value as a starting point without effecting the accuracy of the calibration. The starting offset value will need to be noted when determining the new offset value that is calculated in Step 4. For this example, it will be assumed that 0x00h is used as the starting point.
2. Verify no current is flowing
In most cases, writing 0x00h to the Protection Register to disable the protection FETs will cut off any current flowing into or out of the battery pack. However, some power supplies or loads that may be connected across the battery pack will have small leakage currents that will affect the accuracy of the offset calibration. This leakage can be detected by calibrating the offset with and without the load connected. If a different value for the
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